The NAVSTAR Global Positioning System (GPS) is a satellite-based navigation system developed by the U.S. military in the 1970's. The GPS space segment consists of a nominal constellation of 24 satellites, four satellites in each of 6 orbit planes.
Originally conceived as a navigation aid for ships, the use of the system has become ubiquitous both within the military and within civilian and commercial applications. For example, many cars today are outfitted with GPS navigation systems that locate the car on a displayed digital map to the driver. In commercial applications, GPS systems are used for surveying in addition to controlling vehicles such as graders during the laying of road beds. On these vehicles, the antennas are sometimes located on the blade in addition to the cab. In order to ensure good satellite visibility, however, the antennas must be placed on high poles to provide line of sight to the required four satellites.
The Standard Positioning Service (SPS) signal is currently provided to civilian users of GPS. It is made up of an L-band carrier at 1575.42 megahertz (MHz) (referred to as the L1 carrier) modulated by a pseudorandom noise (PRN) C/A (clear acquisition) code. The satellites are distinguished from each other by their unique C/A codes, which are nearly orthogonal to each other. The C/A code has a chip rate of 1.023 MHz and is repeated every millisecond. A 50 bit per second data stream is modulated with the C/A code to provide satellite ephemeris and health information. The phase of the C/A code provides a measurement of the range to the satellite. This range includes an offset due to the receiver clock and is therefore referred to as the pseudo-range. Since the receiver clock error is common to all satellites, it represents an additional unknown to be solved for along with position. Consequently, to perform a three dimensional position fix, a GPS position detection system traditionally requires a minimum of four satellites (one satellite phase measurement for each of the unknowns). The positioning accuracy provided by the SPS is on the order of ten meters. Due to geometric effects, vertical errors are typically larger than horizontal errors.
Other global positioning systems exist in addition to the NAVSTAR GPS. Within the GNSS (Global Navigation Satellite System) are the Russian GLONASS and the forthcoming European GALILEO GPS systems. Position detection systems can use one or more of these systems to generate position information.
Differential GPS (DGPS) is a variant method for providing higher positional accuracy. If a reference GPS receiver is placed at a known location on the ground, the bulk of the errors associated with the satellite phase measurements can be estimated. Phase corrections can be calculated and broadcast to a roving GPS user. Since most errors are highly correlated in a local area, the roving user's position solution after applying the corrections will be greatly improved.
Traditional DGPS systems use the C/A code phase measurements to arrive at position solutions. These systems provide 95% positioning accuracies on the order of a few meters. The precision of the L1 carrier phase measurement has been used to improve the performance of DGPS. Using carrier smoothed code techniques, DGPS performance improves to the meter level.
Further improvements are achieved through the use of kinematic DGPS. Kinematic DGPS, or differential carrier phase GPS, refers to using the differentially corrected carrier phase measurements, possibly in addition to the code phase. Due to the short wavelength of the L1 carrier phase (about 19 cm), these measurements are extremely precise, on the order of several millimeters. Although the measurements are corrupted slightly by the errors sources, the potential accuracy of kinematic positioning is on the centimeter level. However, the carrier phase measurement has an integer cycle ambiguity associated with it. This ambiguity arises from the fact that each cycle of the carrier phase is indistinguishable from the others; before centimeter level positioning can be achieved, the ambiguity must be resolved.
Some kinematic DGPS systems use a common clock to process carrier signal information from multiple antennas. This allows for position solutions with carrier signals from less than four satellites if relative delays associated with receiving the broadcast phases from the antennas are known. Typically, this delay is determined by measuring the length or delay associated with a fixed length cable that extends between the reference GPS receiver and the slave receiver.
At these precisions, another ambiguity arises from the relationship between the attitude or orientation of the antenna and the nature of the GPS signals. The transmitted GPS signals are righthand circularly polarized (RHCP). Therefore, GPS receive antennas are designed to receive RHCP signals. The measured carrier phase of a circularly polarized signal is a function not only of the distance between the transmit and receive phase centers, but also of the relative orientation of the antennas and particularly the antennas' yaw or rotation about their boresights. Thus, unknowns concerning the orientation or attitude of the antennas can result in ranging errors that become relevant at the resolutions associated with kinematic DGPS.
Traditionally, kinematic GPS applications do not correct for the effects associated with antenna orientation. When the boresights of all of the receive antennas are parallel and constrained to rotate as a single rigid body, the correction is common to all satellites. It, therefore, affects only the differential clock error or line bias, not the position or attitude solution. However, if the yaw angle between antenna boresights becomes large, a RHCP correction should be applied.
One strategy is to find a correction for each transmit and receive antenna pair. The receive antennas are assumed to be flat patch antennas; the results can be generalized for other types of antennas given their off-boresight phase characteristics.
In kinematic GPS applications, the phase measured from one antenna is typically subtracted from that measured at another antenna. For kinematic positioning, the phase measured at the reference station is subtracted from that measured at the roving antenna. For attitude determination, the phase measured at a master antenna is subtracted from those measured at slave antennas. Thus, another method for applying a RHCP correction is to apply a correction to the single differenced phases. This correction is a function of the two receive antenna orientations and the line-of-sight to the transmit antenna. The incoming signal and the receive antennas are assumed to be circularly polarized in the derivation of this correction. Although less general than the previous one, this correction is sufficient for most applications.